Abort crowbar_join if chef-server is not answering on admin server

There is no point in trying to continue, since the chef-client will
fail.

(cherry picked from commit 9cf1c3132c4485ae4d7c893e2bfff674c1b42693)
Skip time synchronization when NTP server is down

This avoids blocking on it forever.

(cherry picked from commit c61f860ddf0ebbff9977befbce753aa0840ef91f)
Avoid infinite loops with sntp

We do it 120 times, and stop looping afterwards.

It is quite a long time in the worst case: timeout seems to be 15
seconds, so that'd be ~30 minutes. However, worst case should be avoided
since we don't even enter the loop if the NTP port is not open on the
server.

If the NTP server replies but without time, that means we try during 2
minutes, which is not too bad.

(cherry picked from commit 57991054344b31b8b35e66191ff3415d272c0cce)
Make crowbar_join work with multiple NTP servers

(cherry picked from commit 970a5a7023e16380518ca228e734784cadb687a6)
Make crowbar_register know about multiple NTP servers

(cherry picked from commit a741f7e3b9775e7e43d70d5fa60ae4a84dacda34)
Make sure that netcat is installed

We add it to the autoyast profile and to crowbar_register, this way
we're 100% sure that it's available for crowbar_join.Make sure

(cherry picked from commit 52838138be902bac548f1b841dd277d43123400a)
crowbar_join: Try longer to ping the admin server

Apparently, on some hardware, the network card takes 20 seconds to be
up, so just one initial ping is not a good enough test. Let's try for
two minutes.

https://bugzilla.novell.com/show_bug.cgi?id=871460
(cherry picked from commit 89be5f01fd790de72d59079cd1176c300a47cd92)
crowbar_join: Get the hostname before pinging the admin server

(cherry picked from commit 7437ddd5327fd95b4fec9dcdd8ed31a0375f286c)
crowbar_join: Do not sleep if we're going to fail anyway

(cherry picked from commit 28add36baf47cb003f94e2c29de6881389c4bc89)
Merge pull request #292 from vuntz/admin-server-down-roxy

Admin server down roxy